(a) Field of the Invention
The invention relates to a universal serial bus device, particularly to an online calibration method and device for a universal serial bus system.
(b) Description of the Related Art
Since the use of universal serial bus becomes popular, various products used as universal serial bus host ends (hereinafter referred to as “USB host end”) and universal serial bus device ends (hereinafter referred to as “USB device end”) are derived. In order to communicate the USB host end and the USB device end, the signal compatibility between the USB host end and the USB device end should be taken into account. A USB device, such as a USB host end or a USB device end, includes a transceiver. The transceiver is used to receive and transmit data between the USB host end and the USB device end and to detect signal of both.
Further during high-speed operation of a USB device with USB 2.0 specification, the USB device transmits signals by a current driving method so that the termination impedance in the transceiver has to be within a tolerable range complied with USB 2.0 specification. Thus the correctness of received signals of the transceiver, and the signal compatibility between the USB host end and the USB device end can be assured.
Deviation often occurs in semiconductor processes. A USB device couples to another USB device where the equivalent termination impedance is referred to any side of a transceiver in the USB device and usually has deviation that exceeds a tolerable range according to the design specification. Therefore, in order to improve a connection compatibility of a USB device with other USB devices, such deviation should be corrected.
A common calibration method is a self-calibration method. Such a method is to use a reference voltage source or current source in a USB device to slightly adjust deviation occurred by a USB device itself to achieve deviation within a designed tolerable range.
However, handshaking of USB devices with USB 2.0 specification during building connection is a process of participation of a USB host end and a USB device end. Since a USB host end and a USB device end are combined together, whole deviation contributed by both ends can not be covered by the separate self-calibration of termination impedance. The compatibility problem during connection may occur or even the functionality of the product may fail. Moreover, the product cannot be correctly connected.